Pn junctions and p-i-n junctions are the key elements of many semiconductor devices including light emitters, rectifying pn diodes, photo detectors, transistors, and thyristors. Semiconductor pn junction consists of two material regions, one of which has n-type electrical conductivity (electron type conductivity), and another region with p-type electrical conductivity (hole electrical conductivity). Conductivity type of semiconductor material can be controlled by the introducing impurity atoms into semiconductor material. For example, n-type conductivity of GaN semiconductor may be caused by the introduction of silicon atoms into GaN, and p-type conductivity of GaN semiconductor may be caused by the introducing of magnesium atoms in GaN. One of widely used technological methods to form pn junction is epitaxial growth of semiconductor material with one conductivity type on the surface of semiconductor material with another conductivity type. In the case of p-i-n junctions, insulating material (i-type material), for example i-type GaN, is located in between p-type and n-type regions.
Compound semiconductors based on III-V nitrides, for example GaN, AlN, AlGaN, or InAlGaN have been consider as promising materials for electronic and optoelectronic devices for long time due to their unique physical and electronic properties, in particular, direct band-gap structure, a high electric breakdown field, and a high thermal conductivity. AlGaN and GaInN material systems form continues alloys, which cover band-gap energy range from 1.9 eV (InN) to 6.2 eV (AlN). These fundamental properties make these materials very attractive for semiconductor electronic applications, including light emitters, photodetectors, microwave transistors. For long time, development of semiconductor devices based on III-V nitride semiconductors has been limited by lack of the material having p-type electrical conductivity. Because of this reason, pn junctions and p-i-n structures based on III-V nitrides did not exist that time. The breakthrough took place on 1989 when the development of p-type GaN was reported, for the first time. Following advance in the epitaxial growth and doping of GaN and AlGaN has paved the way for the development of full set of GaN-based semiconductor devices employing pn junctions or p-i-n junctions. GaN-based light emitter diodes (LEDs), laser diodes, ultra violet (UV) photo detectors, have been demonstrated.
All the above results on the fabrication of III-V nitride pn junctions were obtained using metal organic chemical vapor deposition (MOCVD) technique and, to some extend, by molecular beam epitaxy (MBE).
The most advanced results have been demonstrated by MOCVD technique, which includes the growth of III-V nitride epitaxial layer from vapor phase using Mg as acceptor impurity and the following anneal of the grown structure in order to produce p-type material. In this method, III-V nitride compound semiconductor is grown from the vapor phase using metal organic gases as sources of group III metals, for example trimethylaluminum (TMA) is used as aluminum source and trimethylgallium (TMG) is used as gallium source. Ammonia is usually used as nitrogen source. Growth of III-V nitride semiconductor takes place in reactor chamber on a substrate. During the growth, the substrate is kept at growth temperature ranged from 800 to 1100.degree. C. Single crystal wafers of sapphire or silicon carbide serve as substrates for III-V nitride deposition by MOCVD method. As a result of such MOCVD process, thin layers (usually not thicker than 5 microns) of III-V nitride compound semiconductors can be grown on a substrate. In order to control electrical properties of the grown material, for example type of electrical conductivity, electrically active impurities are introduced in the reaction chamber during the growth. Undoped III-V nitrides usually exhibit n-type conductivity. The value of n-type conductivity can be controlled by introducing Si impurity (in form of SiH.sub.4 gas, for example) in the reaction chamber during the growth. In order to obtain p-type III-V nitride material by MOCVD method, Mg impurity is introduced in the reactor chamber during the growth. Biscyclopentadienylmagnesium (Cp.sub.2 Mg) is used as a Mg source for III-V nitride doping. In order to form a pn junction, first MOCVD growth process is carried out using one type of impurity, for example Si donor impurity, to form n-type layer of III-V nitride semiconductor, and after that the second layer doped with another impurity, for example Mg acceptor impurity, to form p-type layer is grown by MOCVD process. As grown Mg doped material grown by MOCVD is highly resistive, and in order to activate p-type conductivity, high temperature post-growth anneal in nitrogen atmosphere is required. This procedure has been applied to form high quality GaN and AlGaN pn junctions. In the case of p-i-n structure, insulating layer (i-type) of III-V nitride is grown by MOCVD in between p-type and n-type layers. Recent progress in MOCVD technology for III-V nitride compound semiconductors resulted in the commercialization of a number of advanced semiconductor devices, including UV, blue, and green light emitting diodes.
However, the MOCVD technology has a number of limitations.
(1) This is expensive method requiring complicated growth equipment. In order to form III-V nitride pn junctions and p-i-n junctions by MOCVD process, metal organic sources, for example TMA (Al source) and TMG (Ga source) must be used.
(2) Complicated chemical compounds have to be used as acceptor impurity sources, for example biscyclopentadienylmagnesium (Cp.sub.2 Mg) is usually used as a Mg source.
(3) The MOCVD method does not provide a growth rate for III-V nitride growth higher than a few microns per hr (usually less than 3 microns/hr), which leads to long growth runs. For device structures, which require thick layers (for example high voltage rectifier diodes with base region of about 30 microns thick), the MOCVD technology can not be practically employed.
(4) Another disadvantage of the MOCVD method is that n-type AlGaN layers grown by MOCVD are insulating, if AlN concentration is high (&gt;50 mol. %). This fact limits the AlN concentration in III-V nitride layers forming the pn junction.
(5) In order to form high-quality III-V nitride material on SiC substrates, MOCVD method requires to grow a buffer layer in-between SiC and III-V nitride, which makes impossible to fabricate devices utilizing direct contact between SiC and GaN such as SiC/GaN pn junction, particularly n-SiC/p-GaN junction.
(6) Usually, III-V nitride pn junctions grown by MOCVD require post-growth anneal to activate acceptor impurities and obtain p-type material.
There were a number of attempts to develop an alternative epitaxial growth technique to form III-V nitride pn junctions and p-i-n junctions. One method, which has been considered as a promising technique for the fabrication of III-V nitride device structures, is hydride vapor phase epitaxy (HVPE). HVPE (halide-hydride vapor phase epitaxy) means the technique for epitaxy during which the deposition of compound semiconductor (e.g. A.sup.III N or solid solution A.sup.III.sub.x B.sup.III.sub.y C.sup.III.sub.1-x-y N, where x+y&lt;1, and A, B, C are the metal of group III of the periodic system) occured on heated substrate from vapour phase and the metal is transported using metal halide e.g. A.sup.III B.sup.VII or A.sup.III B.sup.VII.sub.3 (GaCl, AlCl.sub.3, AlBr.sub.3 etc).
The HVPE method is convenient for mass production of semiconductor devices due to its low cost, flexibility of growth conditions, and good reproducibility. In this method, Ga metal and Al metal are used as source materials, thus HVPE technology does not require expensive source materials. The principals of the HVPE technology are well-known. Due to the high growth rate (up to 100 microns/hr), the HVPE technique can be used for the deposition of thick GaN layers.
Recently, significant progress has been achieved in HVPE growth of III-V nitride compound semiconductor materials. It was shown that AlGaN and AlN layers, as well as AlGaN/GaN heterostructures can be grown by HVPE technique. The AlGaN alloys grown by HVPE were found to be electrically conductive up to 70 mol. % of AlN. Furthermore, since these GaN layers were grown directly on conducting SiC wafers without insulating buffer layers, diodes with n-GaN/p-SiC heterojunctions were fabricated by HVPE.
Nevertheless, p-type III-V nitride semiconductor has not been produced by conventional HVPE technique, and therefore semiconductor devices utilizing pn junctions and p-i-n structures have not been created. It was shown that Mg metal may be used as a source of Mg acceptor impurity for III-V nitride semiconductor in the HVPE method, thus expensive magnesium source such as biscycropentadienylmagnesium is not required. However, Mg doping in conventional HVPE technique has resulted in the growth of insulating (i-type) III-V nitride materials, not p-type material. All attempts to fabricate p-type III-V nitride materials by HVPE techniques were unsuccessful. As a result, potential advantages of the HVPE technique for III-V nitride materials were not realized for semiconductor devices requiring pn junctions or p-i-n junctions such as light emitting pn diodes, pn diode rectifiers, p-i-n photodetectors, and heterojunction bipolar transistors.
Although, some theoretical explanations for p-type material formation in III-V nitride compound semiconductors are proposed, the mechanisms of this phenomena are not yet entirely clear, and the inventors do not wish to be bound by the theory.
Thus, although the HVPE method offers tremendous potential for the fabrication of III-V compound semiconductor devices, such devices have not been created because of the failure of HVPE technique to produce p-type III-V nitride materials.